(1) Field of the Invention:
This invention relates in particular to an air/fuel ratio controller for an engine, which is equipped with a function to make the air/fuel ratio leaner in a light-load operation zone or the like of the engine.
(2) Description of Related Art:
As one method for improving the specific fuel consumption of an engine, it has been known to burn a lean air-fuel mixture. If the above burning method making use of such a lean air-fuel mixture is applied to a vehicle engine in particular, problems arise that no sufficient power is available upon acceleration in a lean burn period during which the engine power drops unavoidably and good vehicle drivability may not be assured. It has hence been proposed, for example, in Japanese Patent Laid-Open No. 87932/1986 to detect the operation zone of an engine so as to decide whether lean burn should be effected or not and also to detect an accelerated state of the engine so as to make the air/fuel ratio of an air-fuel mixture, which is to be fed to each combustion chamber of the engine, richer for ensuring sufficient engine power during the period of the accelerated state.
Upon detection of an acceleration, it is generally practised as indicated in the above patent publication to discriminate based on an acceleration command by an operator or driver (hereinafter called "driver" collectively) or the rate of a change of the opening rate of a throttle valve whether or not an engine is in an accelerated state or to discriminate based on the rate of a change of the pressure in an intake passage at a point downstream the throttle valve whether or not the engine is in an accelerated state. If the injection quantity of a fuel is increased for the sake of acceleration by the former method, namely, on the basis of the change rate of the throttle valve opening rate, the responsibility in an initial stage of the acceleration is good. The acceleration-related injection-quantity increment is however terminated at the time point of an end of the acceleration command (i.e., at the time point where the change rate of the throttle valve opening rate has reached approximately 0) and the air/fuel ratio is rendered leaner before the actually accelerated state of the engine is terminated (namely, the revolution number of the engine increases sufficiently), resulting in a drawback that the feeling of acceleration is reduced abruptly in a final stage of the actually accelerated state and satisfactory feeling. of driving cannot be obtained. If the injection quantity of a fuel is increased for the sake of acceleration by the latter method, namely, on the basis of the change rate of the pressure in the intake passage at the point downstream the throttle valve, the intake passage acts tentatively as an accumulator for the intake air in an initial stage of the acceleration and a delay takes place with respect to the pressure change. As a result, the initiation of an increment to the injection quantity of the fuel is delayed. As a consequence, the power increment of the engine fails to follow promptly an acceleration command by a driver, leading again to a drawback that no satisfactory feeling of driving is available.